MULTI2 is a block cipher, developed by Hitachi in 1988. Designed for general-purpose cryptography, its current use is encryption of high-definition television broadcasts in Japan.

MULTI2 is a symmetric key algorithm with variable number of rounds. It has a block size of 64 bits, and a key size of 64 bits. A 256-bit implementation-dependent substitution box constant is used during key schedule. Scramble and descramble is done by repeating four basic functions (involutions).

There are a large class of equivalent keys in the Multi2 block cipher. The largest class (so far found) stems from the fact that the Pi3 round function in the key schedule is not bijective. For example, with the following 40-byte input key to the key schedule:

You can perform the following single byte modifications (modification here means XOR against the original key byte):

In this case there are 15 different keys which will schedule to the same 8 32-bit round keys for the ciphers bulk encryption path. The keys are all different in the first keyword used in the Pi3 round function (keys k[1] and k[5]). The collision occurs because a single byte difference turns into a pattern like 0X0X0000 (rotated by 0, 8, 16, or 24 bits) which then expands to a variation of 0X000X00 and finally in the second last line (with the rotate by 16 and the XOR) the differences cancel out. Turning into a zero-delta.

The problem stems from the fact that the function

Where ROL means rotate left by y bits, is not bijective for any value of y. There are similar problems with the Pi2 and Pi4 functions but they are seemingly harder to exploit because the rotation value is smaller.

There are other observations too, for example